Chevrolet Repair: heater blower, windshield defroster, vacuum leaks

Question
I have a 1999 Chevy Van 2500 with a 350. Sometimes the heater or windshield defroster blows fine. Other times I can hear the blower blowing hard but the volume of air is not much. Then sometimes just while driving it may start working and after while stop again. Any ideas where to start looking?
It also has AC but I have no idea if that will work as the temperature around here is freezing. I think when I put the heater on these ports it may do the same thing but I am nit completely sure.

Answer
Hey,

  There are many components to the HVAC system but problems with the mode controller actuator can divert air to the wrong places or put a system in outside recirc.  If it has manual control make sure there is no vacuum leaks to the vacuum diafragm.  The vacuum line comes in from the engine compartment by your doghouse and can get pinched.

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